Emissive displays fall under three categories depending on the type of emissive device in the display: (1) Organic Light Emitting Displays (OLED), (2) Field Emission Displays (FED) and (3) Inorganic Thin Film Electroluminescent Displays (EL). Of these three categories, OLEDs have received the most attention and investment around the world. Approximately 100 companies are developing various aspects of the OLED technology. Commercial OLED products are in the mobile phone and MP3 markets. OLED devices can be made from small molecules (pioneered by Kodak) or polymers (pioneered by Cambridge Display Technology). OLED devices can also be made from phosphorescent materials (pioneered by Universal Display Technology). More than 90% of the commercial products use Kodak's fluorescent small molecule materials. Polymer materials, on the other hand, offer lower cost manufacturing by using solution processing techniques such as spin coating and ink-jet printing. Polymeric materials are expected to offer a cost effective solution for large size (>20″) OLED displays. Phosphorescent materials offer higher efficiencies and reduce power consumption.
OLED displays suffer from several materials based and manufacturing process dependant problems. For example, OLEDs have short lifetimes, loss of color balance over time, and a high cost of manufacturing. The poor lifetime and color balance issues are due to the chemical properties of emissive device in the OLED. For example, it is difficult to improve the lifetime of blue OLEDs because the higher energy in the blue spectrum tends to destabilize the organic molecules used in the OLED. The cost of manufacturing small molecule full color displays is also very high due the need to use expensive shadow masks to deposit red, green and blue materials. Kodak and others have developed white OLEDs by using color filter technology to overcome this problem. However, the use of color filters adds cost to the bill of materials and reduces the quality of display. Some of the main advantages of the OLED display are being taken away by this approach.
Polymeric materials offer a possible route to achieve low cost high volume manufacturing by using ink-jet printing. However, polymers have even shorter lifetimes compared to small molecules. Lifetimes must increase by an order of magnitude before polymer materials can be commercially viable.
The next generation emissive display technology is expected to be based on newly emerging nanomaterials called quantum dots (QD). The emission color in the QDs can be adjusted simply by changing the dimension of the dots. The usefulness of quantum dots in building an emissive display has already been demonstrated in QD-OLED. See Seth Coe et al., Nature 420, 800 (2002). Emission in these displays is from inorganic materials such as CdSe which are inherently more stable than OLED materials. Stable blue materials can be achieved simply by reducing the size of the quantum dots.
Display devices made with QDs have quantum efficiencies which are an order of magnitude lower than OLED. QDs have been combined with OLED materials to improve efficiency. See US2004/0023010. However, this approach produces only modest improvement in efficiency while decreasing the display lifetime and complicating the manufacturing process.